The present invention relates to a pipe made of synthetic resin and intended for underground use where a high pressure must be withstood. Such pipes are typically used for water mains pipes, sewer pipes, drain pipes, electric and telephone cable conduits, and the like.
Pipes intended for such underground use must, in addition to a high pressure-withstanding strength, be light in weight so that they can be easily handled in constructing underground pipelines.
Conventionally, concrete pipes and cast-iron pipes have been used for the purposes described above. These conventional pipes, however, are very heavy, and hence it is quite difficult to handle them in constructing underground pipelines.
Recently, underground pipes made of a synthetic resin material and having a helically configured wall have become available. These pipes are easy to handle because of their light weight. Specifically, because these pipes are light in weight, the individual pipe segments can be made longer than in the case of employing concrete pipes or cast-iron pipes. Also, curving or bending of such pipes during installation can readily be accomplished because of their inherent flexibility. Accordingly, such pipes have lately come into widespread use.
A cross-sectional view through a portion of the wall of one of these pipes is illustrated in FIG. 1. As shown in this drawing, the wall 01 of the pipe has a helical reinforcing corrugation (reinforcing rib) formed on its outside. To increase the pressure-withstanding strength of such a pipe, it has been the practice to merely increase the thickness of the wall 01. However, this approach to increasing the pressure-withstanding strength of the pipe results in a more costly pipe since a larger amount of synthetic resin material must be used in its manufacture. Also, the flexibility of the pipe, which is one of the outstanding features of a synthetic resin pipe, becomes less.